With recent developments in biotechnology, a variety of nucleic acids exhibiting physiologically active functions in cells have been discovered. For example, it is known that siRNA (small interfering RNA) causes the degradation of the mRNA of a target gene in a cell, and interferes with the expression of the target gene (RNA interference). The inhibitory function on target gene expression due to such RNA interference is useful for the mitigation or treatment of disease symptoms caused by the abnormal expression of specific genes or gene clusters, and the development of therapeutic agents using siRNA is expected. However, since nucleic acids are water-soluble macromolecules having a negative charge, they have extremely low intracellular gene delivery efficiencies, resulting in inefficient therapeutic effects under gene therapies including siRNA therapies.
Use of a carrier (vector) has been known to efficiently deliver genes into cells. Vectors include viral vectors and non-viral vectors. Viral vectors have high nucleic acid introduction efficiency; however, there are various unknown safety aspects including pathogenicity, immunogenicity and cytotoxicity. Therefore, the use of non-viral vectors is desired for clinical applications.
Examples of non-viral vectors include Lipofectamine™2000, which is already commercially available. Further, a cationic lipid with a specific structure (see, Patent Document 1), and a composition (see, Patent Document 2) containing an amphiphilic compound and a polycation have been reported. Intracellular nucleic acid delivery using a non-viral vector is conducted by first mixing the nucleic acid to be delivered with the non-viral vector to form a complex, and then bringing the complex into contact with the target cell. When a non-viral vector can form a liposome, the vector is incorporated into a cell with a nucleic acid enveloped in the liposome, thereby conducting intracellular nucleic acid delivery.
Nucleic acids such as siRNA, however, have the specific features of poor stability and a strong electric charge. Therefore, the nucleic acid has problematically reduced stability when mixed with a non-viral vector, inhibiting continuous nucleic acid introduction into a cell. Though an example in which a nucleic acid is entrapped in a liposome by forming a complex of the siRNA and the cationic polymer has been known (see, Non-patent Document 1), it is not usable in practice in view of the cytotoxicity of the cationic polymer. Further, even when a stable complex can be formed using a known non-viral vector and a nucleic acid, the complex may have low intracellular delivery ability or may be rapidly delivered into a cell. Therefore, such known non-viral vectors cannot persistently maintain the nucleic acid in the cell; which fails to keep the desired effects obtained by the nucleic acid.
In light of the prior art, the development of techniques for efficiently delivering a nucleic acid (e.g., siRNA) into a cell and for persistently maintaining the nucleic acid with low toxicity and high safety has been desired.    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-529439    Patent Document 2: Japanese Unexamined Patent Publication No. 2005-508394    Non-patent Document 1: Kentaro Kogure et al., Development of a non-viral-multifunctional-envelope-type nano device by a novel lipid film hydration method, J. Control. Release, 98 (2004) 317-323